I don't know that I would be able to do it in this situation as I may be too stunned in the moment, but I definitely hope I would say something to the wife just to let her know that she's married to man that is most-likely, or going to be, cheating on her. I feel that women should know when their spouses are cheaters and I always feel awful when they don't.

It's ok, at work this guy calls me a bitch, and then a week later says "I'm not hitting on you, I have a girlfriend, but you're breathtaking. your red hair, hazel eyes and braces make you stunning."
Because Braces are hot to middle aged men.